Carbon fibers have started to spread also for industrial applications such as architecture, engineering, and energy related use in addition to the conventional applications such as aircraft and sports gear, with the demand therefor rapidly increased. In order to further accelerate the increase, realization of a carbon fiber of a lower cost is desired. As a representative precursor fiber yarn for producing a carbon fiber, there is an acrylic based fiber yarn, which is widely used. According to the common carbon fiber production, carbon fibers are produced by obtaining flame resistant fibers by a flame resistant process of applying a heating process to acrylic based fiber yarns in an oxidizing atmosphere of 200 to 300° C., and subsequently a carbonizing process of applying a heating process in an inert atmosphere of 1,000° C. or higher. Since the carbon fibers thus obtained have various excellent physical properties, as mentioned above, they are used widely as reinforcing fibers for various kinds of fiber reinforcing composite materials, or the like in many fields.
In general, the acrylic based fiber yarns as the precursor fiber yarns for the carbon fiber production are supplied in a form wound up on a bobbin, or the like, or in a form folded and stacked in a box. Therefore, in order to achieve a low cost and improve the operability of a firing process including a flame resistant process and a carbonizing process, a trailing end of an acrylic based fiber yarn of the aforementioned form needs to be connected with a leading end of another acrylic based yarn for providing a carbon fiber, because it is necessary for continuously transmitting the acrylic based fiber yarns and applying the firing process thereto so as to produce a carbon fiber.
As means for improving the operability in the firing process by continuously supplying the acrylic based yarn fibers in a production process for carbon fibers with connecting the ends, for example, Japanese Patent Application Laid-Open No. 54-50624 discloses a method for applying to a connecting portion of acrylic based fiber yarns a flame resistant compound such as diester oil, silicone oil, halogenated hydrocarbon, and a grease obtained from ore oil and a metal soap. Moreover, Japanese Patent Application Laid-Open No. 56-37315 discloses a method for forming a connecting portion by preliminarily tying the end as a loop of an acrylic based fiber yarn after applying a thermal process, and entangling the same with the loop of another one. Furthermore, the Japanese Patent Application Publication No. 1-12850 discloses a method for forming a connecting portion by entangling ends of acrylic based fiber yarns. Moreover, Japanese Patent Application Laid-Open No. 4-214414 discloses a method for forming a connecting portion by entangling ends of acrylic based fiber yarns, and furthermore, adhering to the connecting portion an oxidization reaction inhibiting agent such as boric acid, ammone sulfamate, sodium sulfite, and urea based compound, respectively.
However, the acrylic based fiber yarns having the connecting portions connected by the methods disclosed in the above publications are not compatible with the production condition for high speed production for carbon fibers having the excellent physical property. This is because the acrylic based fiber yarns having the connecting portions by the above methods cannot stably pass through a step of providing flame resistant fibers by a flame resistant process with high heating temperature and processing tension with respect to the acrylic based fiber yarns, and a step of providing carbon fibers by a carbonizing process with a high processing tension. In particular, in the case of connecting the precursor fibers with each other, burning and thread breakage are generated due to heat accumulation at the connecting portion.
Therefore, for passage of the flame resistant process and the carbonizing process by the acrylic based fiber yarns having the connecting portions by the connecting methods without a problem, the condition of either the flame resistant process with the high heating temperature and processing tension or the carbonizing process with the high processing tension should be alleviated, and thus the carbon fibers can hardly be produced by high speed production.
However, in the case where the acrylic based fiber yarns are connected by merely tying the ends thereof with each other, drastic heat accumulation is caused at the connecting portion in the flame resistant process so that this causes the troubles such as the thread breakage in the subsequent carbonizing process.
Furthermore, for example, Japanese Patent Application Laid-Open No. 10-226918 discloses a method for producing a carbon fiber by connecting precursor fibers for carbon fiber production via a no heat generating connecting medium at a flame resistant temperature by entanglement at the single thread level, and a production device therefor. Gripping means for the precursor yarns and gripping means for the connecting medium exist independently, and moreover, relax gripping portion for each entangling nozzle, that is, a plurality of relax gripping means are provided. Furthermore, each of the relax gripping means comprises a mechanism to be moved independently with each other for providing a predetermined slacking amount to the precursor yarns, and thus it is an extremely complicated mechanism. Moreover, although it is mentioned that a plurality of nozzles are disposed at a predetermined portion for the connecting process over a predetermined length so as to execute bonding by fluid process at each portion, the number of arranged nozzles, or the arrangement interval are not specifically shown.
Thus, according to the prior arts, a connecting portion capable of realizing a certain process passing property with a device having a simple mechanism has not been obtained.
Therefore, an object of the present invention is to certainly obtain a connecting portion having a high process passing property with a simple mechanism in a production device and a production method for carbon fibers so as to achieve continuous operation and improve the firing process operability for achieving a low cost.